1. Field of the Invention
This invention relates to a process for solvent extracting and dewaxing a waxy petroleum oil. More particularly, this invention relates to a relatively low temperature combination extraction-dewaxing process wherein a waxy lube oil stock is simultaneously extracted and at least partially dewaxed in a dilution chilling zone, sequentially followed by a second extraction and a second dewaxing operation to produce wax and useful lube oils of at least two different VI levels and wherein extraction and dewaxing solvent does not have to be removed from the oil as it passes from zone to zone of the process.
2. Description of the Prior Art
It is well known in the art that in order to manufacture lube oils from unrefined, paraffinic lube oil stocks, the undesirable aromatic and polar constituents must be removed from the oil in order to increase the VI and oxidation stability and further, the wax must also be removed in order to increase the fluidity and reduce the pour point. The desired pour point depends on the service conditions for which the final oil is intended. Most methods for producing lube oil stocks comprise (1) distilling a lube oil fraction from a crude oil in an atmospheric or vacuum pipestill, (2) solvent extracting the lube oil fraction in an extraction zone to produce a raffinate phase and an extract phase with most of the solvent and the undesirable aromatic and polar constituents present in the extract phase and the desired lube oil in the raffinate, (3) removing the extraction solvent from the extract and raffinate phases, (4) solvent dewaxing the solvent-free raffinate to precipitate the wax therefrom thereby producing a waxy slurry and (5) separating solid wax from the slurry to produce an oil/solvent mixture containing the desired lube oil.
In many instances it is desired to obtain more than one lube oil stock from a single fraction. This requires multiple extraction and dewaxing operations with the concomitant necessity of removing the extraction solvent from each extracted stream of oil before it is solvent dewaxed. The extraction solvent is invariably removed from the oil by thermal means such as flash evaporation, distillation and stripping. Therefore, each oil/solvent mixture resulting from an extraction operation must be heated up to remove the solvent from the oil and then cooled down prior to entering the cool dewaxing zone. Another disadvantage of conventional extraction-dewaxing processes is the necessity of including scraped-surface chillers in the dewaxing operation unless autorefrigerative dewaxing is used. These chillers are very expensive and many of them are required because they have low throughputs.
In one particular type of solvent dewaxing process, waxy oil and solvent at approximately the same temperature are mixed in such a manner so as to effect complete and thorough solution of the oil and the solvent before being cooled or chilled. This solution is then cooled at a uniform, slow rate in scraped-surface chillers under conditions which tend to minimize agitation of the solution as the wax precipitates out. There is a continual loss of heat exchange capacity in these chillers due to decreasing cooling and heat transfer rate resulting from the deposition of wax on the heat transfer surfaces of the exchangers thereby acting as heat insulation. Further, poor filtration rates result because the wax crystals are mashed and broken by the scrapers inside the chillers. In another method of solvent dewaxing, dewaxing solvent is added to the oil at several points along a chilling apparatus. However, the waxy oil is first chilled without solvent until some wax crystallization has occurred and the mixture has thickened considerably. A first increment of solvent is introduced at this point in order to maintain fluidity, cooling continues and more wax is precipitated. A second increment of solvent is added to maintain fluidity. This process is repeated in scrapedsurface chillers until the desired oil-wax filtration temperature is reached, at which point additional solvent is added in order to reduce the viscosity of the mixture to that desired for the filtration step. In this method the temperature of the incrementally added solvent should also be about the same as that of the wax/oil/solvent mixture. If the solvent is introduced at a lower temperature, shock chilling of the slurry occurs resulting in the formation of small and/or acicula shaped wax crystals with attendant poor filter rate. In both this method and in the one previously described, it is necessary to provide heat exchangers to warm up the dewaxing solvent to the temperature of the oil in order to avoid shock chilling.
It is now well known that the adverse shock chilling effect caused by the incremental addition of cold dewaxing solvent can be overcome by introducing the waxy oil into an elongated, staged cooling zone or tower at a temperature above its cloud point and incrementally introducing a cold dewaxing solvent into said zone, along a plurality of points or stages therein, while maintaining a high degree of agitation so as to effect substantially instantaneous mixing of the solvent and wax/oil mixture as they progress through said zone. The basic concept is shown in U.S. Pat. No. 3,773,650, the disclosures of which are incorporated herein by reference and shall hereinafter be referred to as dilution chilling. This avoids the necessity of having to heat the dewaxing solvent up to the temperature of the waxy oil entering the dewaxing or chilling zone, gives better wax crystals resulting in higher filtration rates and also avoids the need for employing scrapers in the chilling zone, because the wax does not tend to deposit on the surfaces therein since it is the incoming cold solvent that cools the waxy oil in said zone and not cold, heat exchange surfaces. However, even in this process the waxy slurry resulting from the chilling zone must subsequently be cooled down to the wax filtration temperature via the use of scraped-surface chillers.
Therefore, in an extraction-dewaxing process it would be a considerable improvement to the art if one could eliminate the need for (a) using scraped-surface chillers, (b) having to remove the extraction solvent from the extracted oil prior to dewaxing same and (c) having to heat up the dewaxing and extraction solvents prior to mixing them with the oil.